In robotics health care, aeronautics and space applications, tactile sensors are playing an increasingly important role in providing a robot or other pre-programmed automatic controller with the necessary information that will enable the robot or controller to operate in a nonstructured environment. Many industrial applications require robots with precision control and manipulation of a robotic arm. These applications include such areas as automobile assembly, automated semiconductor processing and the like. Tactile sensors can be used to provide a robot arm with a sense of touch, that is, the amount of exerted pressure and/or force and its distribution. Tactile sensors are being increasingly used to supplement the loss of visual information experienced by blind persons. In space and aeronautic applications high-precision tactile sensors are required, as for example, in an "astronaut glove."
For all such applications it is important to have tactile sensors that are stable over time, precise, rugged, and have as high a resolution as may be required. Many existing tactile sensors have shortcomings that limit their ability to be successfully applied in various applications, including instabilities and sensitivities to parameters such as temperature, lack of high resolution, high costs and complex construction and fabrication methods. In light of the foregoing, it is clear that the successful development of a high-performance reliable tactile imager remains an important goal of robotics in spite of a wide variety of reported designs.
Some of the more interesting design approaches have been based upon semiconductor technology. The usual goals to be achieved by using such technology are high density, high resolution and high reliability tactile sensor arrays in a low-cost process. Examples of such approaches are found in M.H. Raibert, "An All-digital VLSI Tactile Array Sensor," Proceedings of the International Conference on Robotics pp 314-319 (Atlanta, Ga.; Mar., 1984); R.A. Boie, "Capacitive Impedance Readout Tactile Image Sensor." Proceedings of the International Conference on Robotics pp. 370-378 (Atlanta, Ga.; Mar., 1984); K. Chun and K.D. Wise, "A High-Performance Silicon Tactile Imager Based on a Capacitive Cell," IEEE Trans Electron Devices Vol 32, pp 1196-1201 (July, 1985). However, most such structures do not lend themselves easily to very large high-resolution arrays and are limited by the size of the imaging cell.
Recently a piezoresistive 32.times.32 element tactile imager using deposited thin film diaphragms was reported. See S. Sugiyama et al , "High-Resolution Silicon Pressure Imager with CMOS Processing Circuits," Transducer '87, pp. 444-447 (June, 1987). Although this device is compatible with the integration of on-chip CMOS circuitry, it requires a complex fabrication process capable of precise control. In particular, the deposited thin film materials employed are known to present problems when deposited to a thickness greater than three microns. Moreover the reproducibility of such thin film processes has historically been difficult to control, and the mechanical strength and integrity of such deposited thin films does not equal that of single-crystal silicon films.
In light of the foregoing problems and shortcomings, it is an object of the present invention to provide a high-performance multi-element capacitive silicon tactile imaging array for use in applications where high density and high resolution are important.
A further object of the present invention is to provide a capacitive tactile imager made using a high-yield fabrication process starting with a single-crystal silicon wafer for excellent strength, uniformity and reproducibility.
Yet another object of the invention is to provide a tactile imager which offers six or more bits of force resolution with a maximum operating force of at least about one gram per element.
Still another object of the present invention is to introduce a new capacitive element structure for tactile sensing which employs a thick center plate for the sense capacitor supported by thinner support beams on either side of the center plate, which can be readily scaled up or down in size and in sensitivity.
One more object is to provide a sensor structure compatible with the fabrication of a tactile imager on thin glass substrates capable of bending and mounting on curved surfaces, thereby mimicking human fingers.